1. Field of the Invention
The present invention relates to a drum brake for use in the brake system of a vehicle such as an automobile, and more particularly relates to a duo-servo type drum brake.
2. Prior Art
A drum brake known as a duo-servo type drum brake typically comprises a backing plate fixedly mounted on a non-rotatable member such as an axle housing of a vehicle, a drum mounted on a rotatable member such as a shaft, a pair of brake shoes expandably mounted on the backing plate by means of respective shoe hold-down units and adapted to be pressed onto the drum when they are expanded, a connecting means connecting lower ends of the respective brake shoes with each other, and an anchor with which the upper ends of the respective brake shoes are engageable. In the duo-servo type drum brake, self-servo operation occurs when the vehicle is advancing in any direction and in reverse, thereby generating a braking force.
It is to be noted that, in the duo-servo type drum brake, the shoe hold-down units only bias the respective brake shoes against the backing plate, and that the lower ends of the brake shoes connected with each other are in a floating condition. As a result, a braking torque generated when the vehicle is being braked, namely a braking force generated when the brake shoes are being pressed onto the drum acts on the anchor. The anchor must, therefore, be fixed securely.
FIGS. 1 to 6 show conventional ways for mounting an anchor on a backing plate. The same reference numerals will be used to designate the corresponding elements in the respective arrangements.
FIGS. 1 and 2 show a first conventional arrangement in which an anchor 1 is provided with a base portion 1a including two extended portions 1b. Shown at 1c is a torque receiving portion of the anchor 1. Shown at 2 and 2a are bolts for fixedly mounting the drum brake on a non-rotatable member of a vehicle such as an axle housing (not shown). The bolts 2 and 2a are respectively located on the lower and upper halves of a backing plate 3. The extended portions 1b respectively extend to the upwardly-located bolts 2a. On the reverse side of the backing plate 3 is mounted a first reinforcing plate 4 which is provided with generally the same shape as that of the base portion 1a. The backing plate 3, base portion 1a and reinforcing plate 4 are fixed together by means of rivets 5 with the backing plate being located between the other two. It is to be noted that a second reinforcing plate 6 is employed in order to reinforce the portions of the backing plate 3, base portion 1a and first reinforcing plate 4 around the rivets 5. In the drum brake with the above-described construction, a braking torque applied to the anchor 1 by the braking shoes 7, 8 is transmitted to the non-rotatable member of the vehicle through the integrated reinforced portion consisting of the backing plate 3, base portion 1a, and first and second reinforcing plates 4, 6, respectively, and through the bolts 2a.
FIGS. 3 and 4 show a second conventional arrangement for mounting an anchor 1 on a backing plate 3 in which upwardly located bolts 2a are located closer to the anchor 1 than in the case of the first-mentioned arrangement. For that reason, a thicker first reinforcing plate 4 is used instead of using a second reinforcing plate 6 in addition to a relatively thinner first enforcing plate as in the case of the first-mentioned arrangement. Rivets 5 are used to fix the backing plate 3, a base portion 1a of the anchor 1 and the reinforcing plate 4 to one another. A braking torque applied to the anchor 1 by brake shoes 7 and 8 is transmitted to a non-rotatable member of the vehicle through the integrated reinforced portion consisting of the backing plate 3, base portion 1a and reinforcing plate 4 and through the bolts 2a.
FIGS. 5 and 6 show a third conventional arrangement for mounting an anchor 1 on a backing plate 3. As best shown in FIG. 6, the anchor 1 is welded to a reinforcing plate 9 which is, in turn, welded to the backing plate 3. The reinforcing plate extends to one of mounting bolts 2a. The backing plate 3 is thicker than those employed in the first and second arrangements mentioned above.
However, drum brakes with the above-mentioned arrangements suffer from the following problems.
In any of the above-mentioned arrangements for mounting an anchor on a backing plate, a braking torque applied to the anchor is transmitted to a non-rotatable member of the vehicle through the backing plate, and a torque receiving portion of the anchor is located far away from bolts for mounting the backing plate on a non-rotatable member of the vehicle. As a result, these arrangements inevitably necessitate the use of reinforcing members for reinforcing the anchor and backing plate, which are fixed to the anchor and/or backing plate by means of rivetting, welding and the like. Namely, the conventional arrangements need many parts, resulting in an increase in manufacturing cost and also in an increase in the weight of the drum brake.